1. Field of the Invention
The present invention relates generally to metallic honeycombs and the ways in which such honeycombs are protected against corrosion. More particularly, the present invention is directed to improving the corrosion resistance of metallic honeycomb that is intended for use in corrosive environments.
2. Description of Related Art
Honeycomb structures are well known and widely used in many applications where a high strength and low weight material is required. The combined features of low weight and strength found in honeycomb structures makes them particularly well suited for use in aircraft and other applications where high strength and low weight are particularly desirable. Honeycomb structures have been made from a wide variety of materials including metals, such as aluminum and aluminum alloys. Such structures are referred to as aluminum honeycomb or aluminum core. When the term “aluminum” is used by itself herein, it is understood to cover both aluminum and aluminum alloys. Composite materials made from resin-impregnated fibers and papers have also been widely used in honeycomb structures.
One common process for fabricating honeycomb structures involves bonding multiple thin sheets of aluminum together along specially oriented node lines. The node lines are offset between the different layers in such a way that a honeycomb structure is formed when the layers are expanded. This type of process is commonly referred to as the “expansion” process. The expansion process is not suitable for fabricating honeycomb structures in some instances where certain types of aluminum are used. For example, sheets that are relatively thick or are made from certain alloys of aluminum are too stiff and cannot be formed into honeycomb structures using the expansion process.
A fabrication process or method commonly referred to as the “corrugation” process has been used to form high strength honeycomb structures in those situations where the expansion process is not suitable. The corrugation process involves initially shaping sheets of aluminum into a corrugated configuration. The corrugated aluminum sheets are then bonded together along node lines to form the final honeycomb.
In both the expansion and corrugation processes, adhesives are typically used to bond the aluminum sheets together. Such adhesives are commonly referred to as “node adhesives”. The surfaces of the aluminum sheets are usually coated with a polymer-based coating or otherwise treated to insure good adhesion between the node adhesive and the sheet. The strength of the bond between the adhesive and the treated aluminum sheets is generally determined by measuring the peel strength of the adhesive. Peel strength is typically measured using a standard procedure as set forth in ASTMD 1781. The test basically involves gluing two 3-inch (7.6 cm) or two 1-inch (2.54 cm) wide strips of metal sheets together and the measuring of the amount of force required to peel the two strips apart.
Achieving maximum peel strength for a given adhesive is an important goal when manufacturing high strength honeycomb. Failure of the adhesive bond between aluminum sheets can result in complete failure of the entire structure. Accordingly, there is a need to provide surface coatings and/or treatments for aluminum that enhance adhesion of the node adhesive to the metal surface.
Aluminum honeycomb is a popular core material for use in the construction of sandwich panels where the honeycomb is “sandwiched” between two sheets of material that are commonly referred to as “skins”. An adhesive is typically used to bond the edges of the honeycomb to the skins. In some cases, the skins are self-adhesive and do not require the use of an adhesive layer. The strength of the adhesive bond between the edge of the honeycomb and the skins is also an important consideration in treating or otherwise coating the aluminum core. Failure of the edge adhesive also can result in failure of the structure.
Aluminum is also usually treated to increase resistance to corrosion. Chromate has been a popular corrosion resistant coating for aluminum that has been widely used. However, there are a number of environmental concerns associated with the use of chromate. A number of other corrosion resistant coatings have been developed for aluminum. For example, U.S. Pat. No. 3,687,882 describes coating aluminum with silane-titanante dispersions. Anodizing the aluminum with phosphoric acid and/or coating the aluminum with epoxy or modified phenolic have also been popular methods for making the honeycomb corrosion resistant.
In the art of preparing aluminum honeycomb, there has been and continues to be a need for surface treatments that not only provide good peel strengths at the nodes and edges, but also provide good corrosion protection, especially when the honeycomb is intended for use in known corrosive environments. Developing a coating system or treatment that provides all of the above features is an important goal in making honeycomb from aluminum that is suitable for use where high strength and light weight is required.